Long-term studies on male guinea pigs exposed in utero to diethylstilbestrol (DES) and estradiol (E) will be carried out to determine if there are residual effects which modify their response to sex hormones. Animals at 3 and 6 months will be challenged with estradiol. The effect of cycling activity and pregnancy will also be studied in experimental and control animals. The ontogeny of the receptor proteins for estrogens will be studied in fetal and newborn guinea pigs. Estrogen-sensitive areas of the genital tract in the adult guinea pig will be investigated.